There's a huge rule in coin collecting: never (aggressively) clean coins. Running it under thew tap to release it from the dirt it was contained in is probably

There's a huge rule in coin collecting: never (aggressively) clean coins. Running it under thew tap to release it from the dirt it was contained in is probably fine, but don't break out anything abrasive or acidic. In the US, many high-value coins are sent through professional authenticators and graders, and they will mark a coin as, say, "Mint State details, but cleaned". that "but cleaned" can knock hundreds or thousands off the sales price.